Perhaps the reason that many players don't queue as tank/heals is that they are anxious and don't feel like taking crap from random players as they learn instances. Perhaps the reason that there are 50 min DPS queues is the same.
So instead of a satchel or whatever, why not buff heals by 50-100%, or reduce incoming tank damage by 25-50%, when the role is in very short supply?
Perhaps that would encourage more tanks and heals to queue and learn the instances and become comfortable with the role, et cetera.